This invention relates to a protective tube for a temperature sensor, such as a thermocouple assembly, which is used for measuring temperatures of a material, such as a molten metal. More particularly, this invention relates to a temperature sensor protection tube having a metal tube for housing a temperature sensor and a refractory material covering the metal tube.
It is often necessary to measure the temperature of molten metals, such as aluminum and steel, during production or other industrial processes. The temperatures encountered in these operations are quite high, typically ranging from 660.degree. C. to 1,540.degree. C. and higher. In addition, the corrosive nature of the metallurgical processes involved present problems which must be addressed in the design of protective apparatus for immersible temperature sensors. Materials such as slag and molten metal encountered in various metallurgical processes are corrosive and abrasive and can dissolve and erode protective devices used to cover and protect immersible temperature sensors.
It has been observed that protective tubes including a metal inner tube having a hollow region containing a temperature sensor and a refractory material casing surrounding the metal inner tube are reliable in operation, and relatively simple and inexpensive to manufacture. For example, U.S. Pat. No. 4,871,263 discloses a protective tube which protects a temperature sensor from a hostile molten metal environment and which allows the temperature sensor to respond rapidly to temperature changes of the material whose temperature is to be measured.
It has also been observed that the metal tube and ceramic refractory material used to construct practical and effective temperature sensor protection tubes expand and contract at different rates with changing temperature. This is because the metal in the inner tube and the ceramic refractory material in the outer casing have different coefficients of thermal expansion. Temperature variations cause dimensional changes in all materials. Most materials tend to expand in size when heated and contract in size when cooled. Within moderate ranges of temperature, the change in length per unit length is practically constant and is called the coefficient of thermal expansion.
Temperature sensor protection tubes are frequently exposed to rapid temperature changes whenever these tubes are immersed into a hot molten metal. The metal tube and ceramic refractory casing in the protective tube are subjected to appreciable changes in temperature. These elements expand and contract axially and radially at different rates and develop internal stresses of considerable magnitude during each cycle of temperature change.
An outer ceramic casing including in a temperature sensor protection tube can rupture if it is not able to expand or contract axially and radially somewhat relative to the other elements in the protection tube as the elements in the tube expand and contract with changing temperatures. For example, if the outer ceramic casing is bonded or fitted to an inner metal tube, it is restrained so as to prevent free thermal deformation. Large temperature changes in a restrained outer ceramic casing can produce critical internal stresses and even lead to rupture of the brittle refractory material used to form the sleeve.
There exists a need for a protective device for a temperature sensor which is able to withstand repeated rapid temperature changes without rupturing the brittle outer ceramic refractory casing. A protective device that is configured to permit an inner metal tube and an outer ceramic casing to expand and contract at different rates would be welcomed in the industry.
According to the present invention, a protective tube is provided for a temperature sensor. The protective tube includes an inner metal tube having a hollow region for containing a temperature sensor, an outer refractory sleeve surrounding the inner metal tube and forming an annular space therebetween, and an intermediate elastic buffer member positioned in the annular space to lie between the inner metal tube and the outer refractory sleeve. The intermediate elastic buffer member is made of a cushion material that deforms during expansion and contraction of the inner metal tube and the outer refractory sleeve with changing temperature to protect the brittle outer refractory sleeve from rupture.
In preferred embodiments, the inner metal tube is a steel pipe and the outer refractory sleeve is a cast ceramic refractory casing. The intermediate elastic buffer is either a ceramic paper wrapping, a ceramic refractory coating, or a preformed fiberglass sleeve. These buffers function to provide an elastic cushion between the metal inner tube and the outer refractory sleeve which permits the tube and sleeve to expand and contract at different rates during immersion of the protective tube assembly to high temperature molten metals and other materials. At the same time, the buffer provides a base on the exterior of the metal inner tube that permits the ceramic refractory sleeve to be cast in place without establishing a bond to or tight fit with the inner metal tube. Essentially, the ceramic refractory sleeve can be retained in place in the protective tube assembly in such a way that it is free to expand and contract at a rate different than the inner metal tube during repeated rapid temperature cycles. Advantageously, the improved protective tube assembly is configured to minimize ceramic rupture problems often caused by expansion and contraction of and internal stresses in elements used to construct the protective tube assembly.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.